1. Field of the Invention
The present invention pertains to improved system and sub-system components for time of flight mass spectrometers which are used for detecting, recording and displaying time of flight mass spectrometry data.
2. State of the Art
For the purposes of this invention, time of flight (TOF) mass spectrometry will be defined as the conversion of an electrically recorded mass spectra into a chemically recognizable form. Useful background information about time of flight mass spectrometers and improvements made therein is taught in U.S. patent application Ser. No. 08/751,509 which is herein incorporated by reference.
A common time of flight mass spectrometer architecture exists which will serve as a basis for the present invention. However, it should be remembered that while the present invention is particularly applicable to the systems of a mass spectrometer, the technology is also applicable to other industries.
FIG. 1 is a block diagram which is provided to show some components of a mass spectrometer as known to those skilled in the art. The components identified as 12 are the electronics associated with mass spectrometer which function either as a transient digitizer or a time to digital converter. The electronics 12 are thus electrically coupled to a time of flight mass spectrometer, also known as a waveform recorder, which is generally indicated at 10. A time of flight mass spectrometer 10 is typically constructed of a chamber having an outer vacuum housing 14 and an inner flight tube 16 shown in cross-section. FIG. 1 shows the inner flight tube 16 along its length. The cross section of the inner flight tube 16 can be circular, square or other appropriate tube shapes as known to those skilled in the art of time of flight mass spectrometry. An input port 18 enables particles (ions) to be injected into the flight tube 16 and accelerated down the length of the flight tube 16 by a combination of a pulser 20 (a.k.a. a pulsed repeller plate) and a series of field defining electrodes 22 which are disposed so as to define a pathway 24 within the vacuum housing 14 for the ions to travel. A particle is accelerated down the flight tube 16 toward a microchannel plate 26 and an anode detector 28 (single or multiple anodes). Particles striking the microchannel plate 26 are then detected as an electrical pulse on the anode detector 28, which in turn causes the anode detector 28 to generate an electrical signal which is processed by the electronics 12 associated with the anode detector 28.
An explanation of the mass spectrometer 10 is incomplete without looking at what subsystems precede and follow the mass spectrometer 10. Specifically, these subsystems include the transportation of particles to the mass spectrometer in a transport subsystem 30, and a data system 32 which receives information from the mass spectrometer 10.
The transportation subsystem of particular interest is an ion transport system 30 for directing ions to the mass spectrometer 10. The ion transport system 30 typically includes a series of RF quadrapoles 52 constructed of individual differential pumping units 50. The differential pumping units 50 are typically required because ions are being transported from a pressurized area to the vacuum housing 14. The pressure surrounding the ions is decreased in stages so that a much larger single pumping unit does not have to be used. Consequently, two differential pumping units 50 are shown to illustrate the concept of gradually decreasing pressure.
To provide additional background information, this application incorporates by reference the information taught in U.S. patent application Ser. No. 08/814,898 filed Mar. 12, 1997, with the title TAPERED OR TILTED ELECTRODES TO ALLOW THE SUPERPOSITION OF INDEPENDENTLY CONTROLLABLE DC FIELD GRADIENTS TO RF FIELDS.
In summary, the above-referenced patent application shows state of the art ion transport systems which include the system shown in FIG. 2A. A system 40 is comprised of four electrodes 42, where one electrode 42 is obscured by another in this view. In FIG. 2A, the path 44 an ion 46 travels is shown as indicated to be generally along with and parallel to a lengthwise quadrapole axis 48 of the electrodes 42. The electrodes 42 are charged with an RF component. The RF component is provided so that ions are confined in the radial direction relative to the lengthwise axis 48 of the quadrapole system 40.
The system 40 shown in FIG. 2A is known as an RF quadrapole 52 because of the four electrodes 42 which generate the RF field for confining ions in the radial direction. However, other multipole electrode configurations are also present in the state of the art, such as six (hexapole) or eight (octapole) electrode systems. All function similarly in that the systems provide confinement in the radial direction. However, for an ion 46 traveling near the axis of the system 40, the effect of higher order RF fields created by a greater number of electrodes is minimal.
FIG. 2B is provided to show that the electrodes 42 (FIG. 2A) are arranged such that they are generally positioned at four corners of a square. This means that the distance from any electrode 42 to the nearest two electrodes is generally equidistant for each of the electrodes.
Generating a DC axial field gradient is useful when it is desirable to accelerate ions axially along the quadrapole axis 48. The DC field gradient is also useful in overcoming drag forces arising from the presence of background gas which may be present along the ion path.
An improvement to the RF quadrapole system 40 described above is the creation of quadrapole pairs which are tilted and/or tapered. This effectively doubles the number of electrodes 42 used in the system 40, as will be explained later.
With the above background in mind, some of the problems with state of the art systems for a time of flight mass spectrometer will now be addressed. This will focus attention on the improvements made to the entire system by the present invention. Beginning with the ion transport system 30, it is often necessary to introduce electrical signals into chambers which are pressurized or have a vacuum therein. Chamber walls are typically constructed of discrete insulating materials interspersed with discrete metallic components which are sometimes used for propagating electrical signals. The opposite is also true that the walls are primarily formed of metal and interspersed with insulating material.
Therefore, it would be an advantage to provide a chamber wall which is comprised of materials which are relatively so much easier to fabricate than the chamber walls of the prior art that the costs of the system are significantly reduced.
In a related system shown in FIG. 3, discrete non-insulating material is typically used in the construction of a vacuum flange comprised of a seal 34 and an O-ring 36 for closing a vacuum chamber 38. Furthermore, if electrical signals are to be introduced into the vacuum chamber 38, metallic, glass or ceramic feedthroughs must be provided as conduits through the seal 34. Without the feedthroughs, any electrical signals applied to the seal would be dispersed and insulated from the vacuum chamber by the O-ring which is typically a rubber-like material. The result is that it is difficult and costly to propagate electrical signals into the vacuum chamber 38.
It would therefore be an improvement over the prior art to provide a vacuum seal which would not disperse electrical signals with which it comes into contact. Furthermore, it would be an advantage to provide a seal which did not require the use of a specialized feedthrough which is costly and might require modification of the seal to install.
Another related problem in the prior art is when a skimmer cone is coupled to chamber walls. A skimmer cone shown generally at 51 in FIG. 1 is utilized for the purpose of preserving a supersonic beam in a differentially pumped vacuum system. Skimmer cones of the prior art are comprised of materials which are discrete from the materials used in the construction of the chamber walls 56. The skimmer cone 56 is also used to propagate electrical signals which are used to, for example, generate electrical fields or used by circuitry within the chamber. However, the propagation of an electrical signal requires creating an electrical pathway which is discrete from the materials used in the chamber wall 56 and the skimmer cone 54. Furthermore, the electrical signals must pass between the skimmer cone 54 and the chamber wall 56.
Therefore, it would be an improvement over the prior art to provide a skimmer cone 54 which is constructed integrally with the chamber wall 56. It would also be an improvement to provide an integrated electrical pathway for propagating an electrical signal from outside the vacuum chamber to the inside of the chamber, and finally to the skimmer cone. It would be a further improvement if this could be accomplished at a substantial cost reduction as compared to the prior art.
While the skimmer cone 56 is described as being improved by fabricating it integrally with the materials of the chamber walls 56, this does not exclude the alternative improvement of providing a discrete skimmer cone 56 which can still be integrated to a separate chamber wall 56 more readily than the prior art, as will be discussed later.
What has been generally implied is that the prior art fails to provide a device or method for easily introducing electrical signals into chambers of various pressures (including a vacuum), without substantial difficulty. The source of the difficulty lies in the situation where the material being used for the chamber walls requires the addition of discrete electrical pathways which must be carefully isolated (or insulated) from other electrical pathways and other components of the transport system 30 (FIG. 1). The materials used for the chamber walls do not lend themselves to being easily modified to carry electrical pathways. Furthermore, it is often the case that many electrical signal pathways must cross to reach various circuitry. Crossing electrical pathways is inherently difficult. In addition, conventional feedthroughs a typically expensive ceramic tubing which must be coupled to a metallic surface. Furthermore, metallic walls will short out signals, and insulating walls will carry no signal at all.
Therefore, it would be an improvement over the prior art to provide chamber walls which can cost effectively serve as conduits for electrical pathways which are easily fabricated so as to be integrated into or on the chamber walls. It would be a further advantage to provide the ability to more easily propagate electrical signals from a source outside the chambers to the inside, or vice versa, regardless of the presence of other electrical signal pathways which must be crossed.
Another problem in the prior art is found in the RF quadrapoles 52 (or other RF multipole systems) which are disposed in the differential pumping units 50. The RF quadrapoles 52 are not highly integrated structures in relation to the differential pumping units 50.
Some additional background which is helpful to understand the construction and use of RF quadrapoles follows. A significant drawback to the RF quadrapoles described above is that in addition to an axial DC electrical field, a quadrapolar DC field is disadvantageously generated. The effect of the quadrapolar DC field is summarized as introduction of mass discrimination. More specifically, mass/charge discrimination occurs in that a narrower range of ions can be transported via electrodes, where the range of ions is determined by the mass thereof. To increase an axial acceleration field, a stronger DC field gradient is required. However, the disadvantage is that increasing the strength of the DC gradient results in a corresponding increase in the undesirable quadrapolar DC field.
While a quadrapolar configuration which only has radio frequency energy applied thereto has a theoretical low ion mass cut-off, there is no high ion mass cut-off. However, the addition of the quadrapolar DC field introduces a high ion mass cut-off. In applications requiring a large passband, this high ion mass cut-off is unavoidable in the prior art. This is because the magnitude and sign of the quadrapolar DC field varies with axial position. Therefore, it is not possible to compensate by superpositioning an additional quadrapolar DC field on the system.
Accordingly, it would be an advantage over the prior art to reduce mass discrimination by eliminating the quadrapolar DC field. It would be a further advantage to be able to manipulate the RF quadrapolar, the DC quadrapolar and the DC axial fields independently of each other using better integrated RF quadrapoles.
It would be another improvement over the prior art to provide RF quadrapoles constructed from materials which can have disposed thereon electrically conductive areas for generating electrical fields. In this way, the shape of the RF quadrapoles is determined by the shape of the electrically conductive areas, and not by the shape of the materials forming the RF quadrapole. It would also be an improvement to provide RF quadrapoles which decrease or eliminate quadrapolar DC fields. It would also be an advantage to generate customized axial voltage gradients by selectively shaping the electrically conductive areas on the insulating materials.
The subject matter above has generally been concerned with the ion transport system 30 (see FIG. 1). Moving to the mass spectrometer 10 itself, more detail about the state of the art for this system 10 is also helpful in understanding how it is improved. It was stated previously that the ions travel down an inner flight tube 16. The inner flight tube 16 is disposed within the outer vacuum housing 14. One problem in the state of the art is that existing flight tubes 16 require field defining electrodes to form the inner flight tube 16. Present day implementations of inner flight tubes 16 are costly and difficult to make because of the particular care which is required in creating a proper conduit for the ions to travel from the pulser 20 to the microchannel plate 26.
It would be an improvement to provide an inner flight tube 16 which is less expensive than the prior art by using materials for the support structure of field defining electrodes which is less expensive and relatively simpler to construct when compared to the prior art.
A related aspect is that exposed insulating materials used for construction of the inner flight tube 16 disadvantageously allow the build-up of electrical charges which create electrical fields which can alter the flight path of ions within the inner flight tube.
It would therefore be another improvement to eliminate exposed surfaces of insulating materials being used to construct field defining electrodes. It would be another improvement if the desired electrical fields propagated by the field defining electrodes could be selectively customized in order to alter the trajectory of ions in a desired manner. It would be another advantage if the electrical charge from ions landing on materials of the inner flight tube 16 could be drained away so as to minimize any disturbance which would otherwise be created from the accumulation of ions.
Another improved component of the mass spectrometer 10 is the microchannel plate 26 which the ions strike after traveling the length of the inner flight tube 16. The microchannel plate 26 is shown in greater detail in FIG. 4. The microchannel plate 26 is a actually an array of electron multipliers (called channels 28) oriented in parallel to each other. The channel matrix 28 is usually fabricated from a lead glass which is treated in such a way so as to optimize the secondary emission characteristics of each channel 28 and to render the channel walls semiconducting so as to allow charge replenishment. Thus, each channel 28 can be considered to be a continuous dynode structure which acts as its own dynode resistor chain. Parallel electrical contact to each channel 28 is provided by the deposition of a metallic coating, usually Nichrome or Iconel, on the front and rear surfaces, which then serve as input and output electrodes.
The mounting structure for the microchannel plate 26 typically consists of clamping the microchannel plate 26 between metal "picture frames" 58 as shown in FIG. 5. This structure makes it difficult to make connections with electrical contacts to receive information therefrom because of all the metal used in construction. Furthermore, the metal "picture frames" 58 disadvantageously created hard connection points which could lead to damage of the microchannel plate 26.
It would therefore be an advantage to provide a mounting structure which provided a softer mounting structure to prevent damage to the microchannel plate 26. It would be another advantage to provide a more versatile mounting structure which could provide a plurality of electrical contacts, as well as serving as a mounting structure for additional circuitry in a simple, yet compact structure. Still another advantage would be to integrate insulating and conductive areas in the same mounting structure.
Finally, it would be a further advantage to accomplish signal coupling using buried capacitors which are implemented using electrical traces disposed within a multi-layer printed circuit board.